1. Field of the Invention
The present invention relates generally to an electrical power supply having a plurality of outputs and more specifically to an overload protection circuit.
2. Description of the Related Art
Switched-mode power supplies having a plurality of output voltages are often implemented by means of transformers which contain a plurality of output windings. Since in the case of this principle only one output voltage can be regulated directly via the converter, the remaining outputs have to be regulated indirectly. For this purpose, linear readjusters are used which bring the unregulated voltage UA.sub.X of an output winding to the desired value UA.sub.X . This principle is represented in FIG. 1.
In general, the readjusting circuit contains a current limiter which, in the event of a fault, such as in the event of a short circuit, protects the connected load and the readjuster itself. In most applications, a virtually vertical current limiting characteristic curve is desired. A profile of such a current limiting characteristic curve is represented in FIG. 1a. The associated power loss P.sub.v, which is converted at the linear regulator as a function of the load current, is represented in a second characteristic curve I.sub.load /P.sub.V in FIG. 1b. For the purpose of simplification, for this representation the unregulated output voltage UA.sub.X is assumed as a constant U.sub.0.
As can be seen from the course of the characteristic curve, the power loss rises sharply in current-limiting operation. A series transistor, which is integrated in the linear regulator, must therefore be protected against overheating, that is to say thermal destruction.
In order to avoid the overheating of the series transistor, an over-dimensioned heat sink has previously been used or a switching-off of the circuit arrangement in the case of overload has been obtained.
If the maximum power loss occurring P.sub.max =U.sub.0 .times.I.sub.KS is sufficiently small, this can easily be dissipated by means of a somewhat over-dimensioned heat sink. The series transistor, which converts the power loss into heat, can thus be protected effectively.
For the case in which P.sub.max is relatively large, the simple solution using the over-dimensioned heat sink is no longer tolerable, since this approach would lead to a large-volume and expensive heat sink.
Using a temperature sensor (for example an NTC or bimetallic switch), the temperature of the transistor can be monitored. If the temperature exceeds a limiting value, the readjusting circuit is switched off until it has cooled down once more to a sufficient extent. This procedure is repeated until the cause of the overheating, which can be a short circuit in the load circuit, for example, has been eliminated.
One significant disadvantage of this protection circuit is that a sensor has to be thermally coupled to the transistor which is to be protected in the readjusting circuit, that is to say the two components must be mounted close alongside each other on a heat sink. Since the mounting cannot be carried out in an automated fashion, this solution is relatively expensive and, in addition, requires space on the heat sink.
A method which is often used to protect the transistor arranged in the readjusting circuit from overload is to make the desired value of the current regulation dependent on the output voltage. The advantage of this principle is that the short-circuit current is considerably smaller than the current at which the limiting sets in. Since at a relatively small output voltage, that is to say a relatively large voltage across the transistor, the current is reduced, the power loss is considerably smaller than in the case of irreversible current limiting. However, this method is associated with the disadvantage that it cannot be employed in an unrestricted manner.
In the case of loads which need a constant current, such as for example remotely-fed telephones, it may be that the output voltage cannot "run up". This effect occurs when at the beginning of running up (UA.sub.X =0), the connected loads require a higher load current than the short-circuit current. Since this does not permit a reversible characteristic curve, the loads attempt to establish the desired current by lowering their load impedance. The result is that the loads pass into the short-circuit current range, and the readjuster never leaves the short-circuit point.